Field of the Invention
The present invention relates to an outer rotor type dynamo in which ring-like magnets rotate on the outside of a coil.
Description of the Related Art
Heretofore, a hub dynamo is known as an outer rotor type dynamo to be disposed in a wheel of a bicycle. In the hub dynamo, ring-like magnets are provided on the outside of a coil fixed to the shaft of the wheel. By the rotation of the magnets with the wheel, an induced voltage is generated in the coil. Around the coil, a claw pole-shaped stator yoke is provided. The stator yoke is configured by laminating electromagnetic steel plates (Japanese Patent Laid-Open No. 2001-37108 and Japanese Patent Laid-Open No. 2004-260921).
In the claw pole-shaped stator yoke, magnetic fluxes are concentrated on the root of each tooth portion extending in the axial direction. Therefore, each tooth portion is designed to have a shape in which the root is thick and the tip is thin. Therefore, the magnetic fluxes decrease on the tip portion of each tooth portion, so that an induced voltage generation efficiency decreases. In an aspect in which the stator yoke is not configured from laminated steel plates, an eddy current is likely to increase on the root side of each tooth portion.
Japanese Patent Laid-Open No. 2001-37108 discloses an aspect in which a stator yoke is configured from laminated steel plates. With the configuration disclosed in Japanese Patent Laid-Open No. 2001-37108, an eddy current is difficult to increase in each tooth portion. However, the size on the shaft side of the stator yoke (inner side in the radial direction) is shorter than the size on the outer side in the radial direction, and therefore the number of the electromagnetic steel plates which can be laminated is limited. Moreover, since the interval of each tooth portion is large on the outer side in the radial direction, an increase in the number of magnetic poles of a magnet facing each tooth portion is difficult to achieve. Furthermore, when the electromagnetic steel plates configuring the stator yoke have the same shape, the distance between the end surface of the electromagnetic steel plates and the magnets on the center side in the circumferential direction of the tooth portions which is the lamination direction of the electromagnetic steel plates is longer than the distance between the end surface and the magnets on both sides in the circumferential direction of the tooth portions. As a result, the induced voltage generation efficiency decreases.